The present invention relates to safety devices for use with power operated cargo hoisting equipment and particularly to devices which eliminate the hazards involved when the load hook is overwound to the extent that it engages the eye of the pulley.
During the operation of power driven load hoisting equipment, the winch operator must control the upper limit of movement of the load hook to stop the hook before it is "two-blocked," which is that condition which occurs when the hook is drawn into the eye of the pulley. If, through inadvertence or carelessness on the part of the winch operator, the load hook becomes "two-blocked," then either of two results may occur. The line may break and the load is dropped or, if the line does not break, the overhead boom upon which the pulley is mounted will collapse and both the boom and the load will fall. When either of these results occur, the safety of personnel in the area is greatly endangered and the load being hoisted is usually damaged. If the hoisting operation is being conducted over the side of a vessel, such as when raising oceanographic research instrumentation, the instrument package is lost over the side of the vessel when the line parts upon "two-blocking." In the past, various attempts have been made to prevent "two-blocking" by providing electrical switches which automatically control the extent to which a load may be hoisted. These switches operate to cut off the power to the winch when the load reaches a predetermined distance from the pulley. The electrical switches of this type have not been proven to be sufficiently reliable especially when used on shipboard load hoisting equipment where the switches are subjected to a corrosive salt spray which attacks the switch contacts and eventually renders the switch inoperative.
Further, this invention is an improvement over U.S. Pat. No. 3,446,484 which is directed to a fail-safe cable cutter device for load hoisting equipment. In this patent, the safety device must be threaded onto the cable before attaching the object to be hoisted which requires considerable lost time especially where the object has already been attached to the cable necessitating disconnecting and reconnecting the cable. Further, the fail-safe device is secured to the cable by use of a potting compound or a mechanical clamp, not shown in the patent.